The development of high-current, short-duration pulses of electrons has been a challenging problem for many years. High-current pulses are widely used in injector systems for electron accelerators, both for industrial linacs as well as high-energy accelerators for linear colliders. Short-duration pulses are also used for microwave generation, in klystrons and related devices, for research on advanced methods of particle acceleration, and for injectors used for free-electron-laser (FEL) drivers.
The difficulty of generating very high-current pulses of short duration can be illustrated by examination of a modern linac injector system. A good example is the system designed and built for the Boeing 120 MeV, 1300 MHz linac, which in turn is used as a FEL driver [J. L. Adamski et al., IEEE Trans. Nucl. Sci. NS-32, 3397 (1985); T. F. Godlove and P. Sprangle, Particle Accelerators 34, 169 (1990)]. The Boeing system uses: (a) a gridded, 100 kV electron gun pulsed with a 3-nanosecond pulse; (b) two low-power prebunchers, the first operating at 108 MHz and the second at 433 MHz; and (c) a high-power, tapered-velocity buncher which accelerates the beam bunches up to 2 MeV. The design relies on extensive calculations with computer codes such as EGUN and PARMELA. A carefully tapered, axial magnetic field is applied which starts from zero at the cathode and rises to about 500 Gauss.
With this relatively complex system Boeing obtains a peak current up to about 400 A in pulses of 15 to 20 ps duration, with good emittance. The bunching process yields a peak current which is two orders of magnitude larger than the electron gun current. Space charge forces, which cause the beam to expand both radially and axially, are minimized by using a strong electric field in the high-power buncher, and finally are balanced by forces due to the axial magnetic field. The performance achieved by Boeing appears to be at or near the limit of this type of injector.
During the last few years considerable effort has also been applied to the development of laser-initiated photocathode injectors [J. S. Fraser and R. L. Sheffield, IEEE J. Quantum Elec. QE-23, 1489 (1987); Proc. 9th Int'l FEL Conf., ed. P. Sprangle, C. M. Tang, and J. Walsh, North Holland Publishing, Amsterdam, (1988). R. L. Sheffield, E. R. Gray and J. S. Fraser, p.222; P. J. Tallerico, J. P. Coulon, LA-11189-MS (1988); P. J. Tallerico et al, Linac Proc. 528 (1989); M. E. Jones and W. Peter, IEEE trans. Nucl. Sci. 32 (5), 1794 (1985); P. Schoessow, E. Chojnacki, W. Gai, C. Ho, R. Konecny, S. Mtingwa, J. Norem, M. Rosing, and J. Simpson, Proc. of the 2nd European Particle Accel.Conf. (1990), p. 606]. The best of these have somewhat higher brightness than the Boeing injector, but the reliability depends on the choice of photocathode material, with the more reliable materials requiring a larger laser illumination.